Appropriately substitute symbols in switch conditions

Closes #4557

crates/compiler/mono/src/ir.rs

@@ -7079,10 +7079,18 @@ fn substitute_in_stmt_help<'a>(
             default_branch,
             ret_layout,
         } => {
-            let opt_default = substitute_in_stmt_help(arena, default_branch.1, subs);
-
             let mut did_change = false;
 
+            let cond_symbol = match substitute(subs, *cond_symbol) {
+                Some(s) => {
+                    did_change = true;
+                    s
+                }
+                None => *cond_symbol,
+            };
+
+            let opt_default = substitute_in_stmt_help(arena, default_branch.1, subs);
+
             let opt_branches = Vec::from_iter_in(
                 branches.iter().map(|(label, info, branch)| {
                     match substitute_in_stmt_help(arena, branch, subs) {
@@ -7119,7 +7127,7 @@ fn substitute_in_stmt_help<'a>(
                 };
 
                 Some(arena.alloc(Switch {
-                    cond_symbol: *cond_symbol,
+                    cond_symbol,
                     cond_layout: *cond_layout,
                     default_branch,
                     branches,

crates/compiler/test_mono/generated/issue_4557.txt

@@ -0,0 +1,56 @@
+procedure Bool.11 (#Attr.2, #Attr.3):
+    let Bool.24 : Int1 = lowlevel Eq #Attr.2 #Attr.3;
+    ret Bool.24;
+
+procedure Bool.4 (#Attr.2, #Attr.3):
+    let Bool.23 : Int1 = lowlevel Or #Attr.2 #Attr.3;
+    ret Bool.23;
+
+procedure Test.1 (Test.2, Test.3):
+    let Test.17 : {Int1, Int1} = Struct {Test.2, Test.3};
+    let Test.34 : Int1 = StructAtIndex 0 Test.17;
+    let Test.33 : Int1 = StructAtIndex 1 Test.17;
+    let Test.19 : Int1 = CallByName Test.1 Test.33 Test.34;
+    let Test.27 : {} = Struct {};
+    joinpoint Test.28 Test.21:
+        let Test.23 : {} = Struct {};
+        joinpoint Test.24 Test.22:
+            let Test.20 : Int1 = CallByName Bool.11 Test.21 Test.22;
+            dec Test.22;
+            dec Test.21;
+            let Test.18 : Int1 = CallByName Bool.4 Test.19 Test.20;
+            ret Test.18;
+        in
+        switch Test.33:
+            case 0:
+                let Test.25 : Str = CallByName Test.9 Test.23;
+                jump Test.24 Test.25;
+        
+            default:
+                let Test.26 : Str = CallByName Test.11 Test.23;
+                jump Test.24 Test.26;
+        
+    in
+    switch Test.34:
+        case 0:
+            let Test.29 : Str = CallByName Test.9 Test.27;
+            jump Test.28 Test.29;
+    
+        default:
+            let Test.30 : Str = CallByName Test.11 Test.27;
+            jump Test.28 Test.30;
+    
+
+procedure Test.11 (Test.36):
+    let Test.37 : Str = "a";
+    ret Test.37;
+
+procedure Test.9 (Test.39):
+    let Test.40 : Str = "a";
+    ret Test.40;
+
+procedure Test.0 ():
+    let Test.38 : Int1 = false;
+    let Test.35 : Int1 = true;
+    let Test.13 : Int1 = CallByName Test.1 Test.38 Test.35;
+    ret Test.13;

crates/compiler/test_mono/src/tests.rs

@@ -2242,3 +2242,17 @@ fn lambda_set_with_imported_toplevels_issue_4733() {
         "###
     )
 }
+
+#[mono_test]
+fn issue_4557() {
+    indoc!(
+        r###"
+        app "test" provides [main] to "./platform"
+
+        isEqQ = \q1, q2 -> when T q1 q2 is
+            T (U f1) (U f2) -> Bool.or (isEqQ (U f2) (U f1)) (f1 {} == f2 {})
+
+        main = isEqQ (U \{} -> "a") (U \{} -> "a")
+        "###
+    )
+}